Today I learned that water pepper (Persicaria hydropiper, L. 1800) is otherwise called "arse-smart" in English:
The Characters are;
It is a Plant with an apetalous Flower having several Stamina or Chives which arise from the multisid Calyx: the Pointal afterward becomes an oval-pointed smooth Seed, inclosed in the Capsule, which was before the Flower-cup: to which may be added, It hath jointed Stalks, and the Flowers are produced in Spikes.
—The Gardeners Dictionary (1754)
I failed to find its etymology in OED or other sources.
What could be the etymology of this name?